You have a 3D CAD model. You need functional plastic parts. Injection molding is too expensive and too slow for where you are in the development cycle.
Vacuum casting resin is the answer.
But not all resins are the same. Pick the wrong one, and your prototype fails—not because the part was designed badly, but because the material wasn’t right for the application.
At Yanmee Technology, we work with engineers across the board—design engineers validating concepts, manufacturing engineers planning production, and product development teams bridging the gap to full-scale manufacturing.
This guide walks through the most common vacuum casting resins, what they actually replace, and how to choose the right one for your project.
What Is Vacuum Casting Resin?
Simply put: a two-component liquid polyurethane that cures at room temperature in a silicone mold.
The vacuum step isn’t optional. We pour resin under vacuum to pull out trapped air—without it, you get bubbles trapped in corners and fine details that don’t fill properly. Skip the vacuum, and the part comes out with voids. With it, you get a solid part that looks and performs like injection-molded plastic.
And here’s a distinction that comes up a lot: “vacuum casting” describes the process. “Urethane casting” describes the material. People use them interchangeably, but they’re technically different things. If you’re reading datasheets, knowing which one you’re actually looking at saves a lot of confusion.
Resin Types: Matching Polyurethane to Production Plastics
In vacuum casting, we don’t use true thermoplastics like injection molding. We formulate two-component polyurethane (PU) resins to replicate their mechanical and thermal behavior.
What does that look like in practice? Here’s a direct comparison:
| Production Material | Recommended PU Resin Type | Key Grades We Use | Critical Engineering Spec |
|---|---|---|---|
| ABS (General Purpose) | Rigid PU (ABS-Like) | Hei-Cast 8150 / Sika PX226 | Flexural Modulus ~2,200 MPa. Excellent dimensional stability for enclosures. |
| PC (Transparent) | Optical Polyurethane | Sika PX523 / Hei-Cast T0387 | 90%+ total light transmittance. Requires post-cast polishing for optical clarity. |
| PP / HDPE | Tough Polyurethane | Hei-Cast 8400 / Sika PX215 | High elongation at break. Suitable for snap-fits and living hinges. |
| TPU / Rubber | PU Elastomer | Axson PX265 / Sika PX8260 (Shore 40A–90A) | Configurable Shore hardness. High tear strength for gaskets, seals, and overmolding. |
| PA66 / PBT (High-Temp) | Glass-Filled / High-HDT PU | Sika PX521HT | Heat Deflection Temperature (HDT) up to 120°C (248°F) after post-curing. |
| Flame-Retardant | UL94 V-0 Rated PU | Sika PX8263 | Meets UL94 V-0 standard. Used in electronics and automotive interior components. |
Important note: The PU8260 mentioned above is an elastomer (rubber-like, Shore 80A–85A), not a rigid ABS-like resin. This is a common confusion on datasheets. If you need a soft, flexible part, you want an elastomer. If you need a hard shell, you want a rigid PU. Make sure your supplier clarifies which one they’re quoting.
Critical Selection Factors Beyond the Datasheet
Datasheets give you numbers. Experience tells you what those numbers actually mean.
Here’s what we look for at Yanmee when recommending a resin:
1. Shrinkage Rate
Polyurethane resins shrink during curing—typically 0.5% to 1.0% depending on the formulation. If you’re working with tight tolerances or press-fit features, shrinkage is the first thing you need to think about. The resin shrinks as it cures—it’s just what it does. We can compensate by oversizing the master pattern, but we need to know your tolerance targets before we start cutting. Tell us what you need, and we’ll tell you what’s achievable.
2. Wall Thickness Constraints
Ideal wall thickness: 2.0mm to 4.0mm. Sections thicker than 8mm create internal exothermic heat during curing—which accelerates mold degradation and can cause internal voids. If you have thick sections, we’ll recommend a different resin or a design adjustment.
3. Heat Deflection Temperature (HDT) vs. Glass Transition (Tg)
HDT is the temperature at which a material deforms under load. Tg is where it starts to soften. For most applications, HDT is the number that matters. For PX521HT, HDT is 120°C (248°F)—but only after post-curing. Without post-curing, it’s closer to 80°C. Make sure you’re comparing apples to apples.
4. Post-Curing Requirements
Some resins require post-curing at elevated temperatures to reach their full mechanical properties. Others cure at room temperature and are ready to ship. Post-curing adds 4–12 hours and affects lead time. Factor that into your timeline.
Vacuum Casting vs. Injection Molding: Quick Decision Matrix
| Factor | Vacuum Casting (PU Resin) | Injection Molding (Thermoplastic) |
|---|---|---|
| Tooling cost | $200–$3,000 (silicone) | $5,000–$50,000+ (steel/aluminum) |
| Tooling lead time | 3–7 days | 4–8+ weeks |
| Time to first part | 5–10 business days | 4–8 weeks |
| Tool life | 15–25 parts per mold | 100,000+ cycles |
| Ideal volume | 1–500 parts | 1,000+ parts |
| Material options | PU-based resins only | Full range of thermoplastics |
Realistic Tool Life (What We Actually See)
One of the most misquoted numbers in vacuum casting is mold life.
Here’s the reality: a single silicone mold produces 15 to 25 parts for standard rigid PU resins.
Why so few? Polyurethane curing is exothermic—it generates heat as it cures. That heat, combined with the chemical interaction between the resin and the silicone, degrades the mold surface over time. After 15–25 casts, surface detail starts to fade and dimensional accuracy drifts.
For volumes above 25 parts: We create multiple silicone cavities from the same CNC-machined master pattern. Need 100 parts? We make 4 molds and run them in parallel. That’s how we scale without sacrificing quality.
For volumes above 500 parts: That’s when we start recommending rapid aluminum tooling or transition to injection molding. Vacuum casting isn’t the right fit for production volumes—and we’ll tell you that upfront.
Common Applications by Resin Type
Rigid PU (ABS-like / PC-like):
- Electronic enclosures and housings
- Automotive interior bezels and covers
- Medical device shells
- Drone frames and components
PU Elastomer (Rubber-like):
- Custom gaskets and seals
- Overmolded grips and handles
- Shock-absorbing bumpers
- Flexible connectors
Optical PU (Transparent):
- Light pipes and lens covers
- Clear display housings
- Fluid flow visualization parts
High-Temp PU:
- Under-hood automotive testing parts
- Electronics requiring solder reflow testing
- Components near heat sources
Frequently Asked Questions
What’s the difference between vacuum casting resin and injection molding plastic?
Vacuum casting resin cures at room temperature in a silicone mold. Injection molding uses molten thermoplastic in steel tooling. The trade-off is obvious once you see the numbers: $200–$3,000 for a silicone mold versus $5,000–$50,000+ for steel. So vacuum casting makes sense for 1 to 500 parts. Above that, injection molding usually wins on unit cost.
Can vacuum casting resin produce transparent parts?
Depends on the resin. Optical-grade polyurethanes like PX523 hit around 90% light transmittance—close to glass or clear polycarbonate. But you won’t get that straight out of the mold. Post-casting polishing is usually required to hit optical clarity. If you need a functional clear prototype, it’s doable. If you need a production-grade optical lens, this isn’t the right process.
What surface finish can I expect?
The short answer: whatever you put into the master pattern comes out in the cast part. Polish the SLA or CNC master to a glossy finish? The urethane part comes out glossy. Need a specific texture like Mold-Tech MT-11010? That transfers too. The fidelity is excellent—you’re essentially making a copy of the master surface.
What about overmolding threaded inserts?
We do this regularly. Place the insert in the mold, pour the resin around it, and it cures in place. Works for brass inserts, steel bushings, even some metal brackets. Saves you a secondary tapping operation and ensures alignment. We’ll review your insert geometry and position before committing—some orientations are easier than others.
How do I choose between ABS-like and PC-like resin?
Most functional prototypes don’t need polycarbonate-level performance. ABS-like is cheaper, easier to post-process, and works fine for general enclosures. Use PC-like only if you need optical clarity or significantly higher impact resistance. If you’re not sure, start with ABS-like and test it—you can always step up to PC-like on the next iteration.
What’s the typical lead time?
Count on about a week and a half. Master pattern fabrication runs 2–5 days depending on complexity. Silicone mold curing takes another 1–2 days. First article casting is 1–2 days. So total: 5–10 business days for first samples. Expedite is possible if you’re in a rush—just let us know upfront and we’ll see what’s realistic.
Ready to Start Your Project?
📩 Email your CAD file to info@yanmee.com and we’ll recommend the right vacuum casting resin within 24 hours.
📞 Or book a 15-minute technical call with our engineering team.
Get the right material, the right process, and the right partner—the first time.
About Yanmee Technology
Yanmee Technology is a custom urethane and CNC machining manufacturer serving industrial OEMs, automotive Tier 1/2 suppliers, and medical device engineers.
We bridge the gap between domestic prototyping costs and full-scale production—making sourcing transparent, engineered, and risk-managed.
